Seal for worm gear speed reducer

ABSTRACT

The performance of a worm assembly seal is improved by (1) the use of a hardened steel worm with a bronze worm gear; (2) the use of a hardened steel worm and a bronze worm gear with a tribological coating on one or both of the worm thread or spiral and the gear teeth; 3) the use of a hardened steel worm and a bronze worm gear with a tribological coating on one or both of the input or output shaft or shaft sleeves; or (4) the use of a hardened steel worm with a bronze worm gear with a tribological coating applied to the worm thread, the gear teeth, the input and output shafts or shaft sleeves, or combinations thereof. In each of the three approaches, the worm, worm gear, and input and output shafts or shaft sleeves can have surface finishing treatments. Additionally, in each of the three approaches, carbon or carbon/nitrogen concentration gradients can be added to the worm. As can be appreciated, each approach builds on the prior approach to further improve seal performance, thereby reducing seal leakage and extending seal life.

CROSS-REFERENCE TO RELATED APPLICATIONS

Not Applicable.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

Not Applicable.

BACKGROUND OF THE INVENTION

This invention relates to worm-gear speed reducer assemblies, and, inparticular, to an improved seal for the speed reducer's shafts.

The relative commercial usefulness of worm gear systems is based in partupon the ability for the gearbox seals to prohibit the egress oflubricants; the ability for the gearbox seals to prohibit the ingress ofdebris contaminants; the ability for the gearbox seals to minimize heatgeneration; and the ability for the gearbox seals to minimize wear ofthe seal itself and/or its mating shaft. Hence, the seals of wormgearboxes play an important part in the commercial usefulness of wormgear systems.

However, currently seals for worm gearboxes have a much lower life thanany other gearbox component. Lubricant leaking indicates seal failure.Leaking lubricants into the environment is generally unacceptable andreplacing seals is time consuming and costly. Debris tends to scratchand damage the seal elastomer and/or abrade and wear the shaft or shaftsleeve. The seals are also adversely affected by chemical reactions withthe lubricant, by wear imposed by contact with the shaft or shaftsleeve, and by heat that can embrittle the elastomer. The seal is likelyto be replaced one or more times over the useful lifetime of thegearbox.

BRIEF SUMMARY OF THE INVENTION

A worm gear assembly comprises a worm having a worm shaft and a spiralthread, a worm gear having worm gear teeth, and a worm shaft extendingfrom the center of the worm gear. The worm spiral and the worm gear arecontained within a housing in a meshing relationship. The worm and gearshafts extend from the housing and are supported in walls of the housingby bearings. Seals at the housing wall extend between an opening in theworm gear housing and the worm and gear shafts. The seals prevent theegress of lubricant and the ingress of debris. A seal-shaft interface isformed where the seals contact the worm and gear shafts or shaftsleeves. We have found that the seal performance can be enhanced (i.e.,the useful life of the seal can be increased) by (1) using of a hardenedsteel worm with a bronze worm gear; (2) using of a hardened steel wormand a bronze worm gear with a tribological coating on one or both of theworm thread or spiral and the gear teeth; 3) the use of a hardened steelworm and a bronze worm gear with a tribological coating on one or bothof the input or output shaft or shaft sleeves; or (4) the use of ahardened steel worm with a bronze worm gear with a tribological coatingapplied to the worm thread, the gear teeth, the input and output shaftsor shaft sleeves, or combinations thereof. In each of the approaches,the worm, worm gear, and input and output shafts or shaft sleeves canhave surface finishing treatments. Additionally, in each of theapproaches, carbon or carbon/nitrogen concentration gradients can beadded to the worm.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 is a cross-sectional view of a speed-reducer assembly includingseals of the present invention;

FIG. 2 is an enlarged cross-sectional view taken along circle 2 of FIG.1 showing the seals in greater detail; and

FIGS. 3 a-d are graphs showing illustrative profiles of carbon andnitrogen profile of worms of the present invention when carburized (FIG.3 a), nitrided (FIG. 3 b), carbonitrided (FIG. 3 c), and nitrocarburized(FIG. 3 d). The illustrative profiles are not to scale.

Corresponding reference numerals will be used throughout the severalfigures of the drawings.

DETAILED DESCRIPTION OF THE INVENTION

The following detailed description illustrates the invention by way ofexample and not by way of limitation. This description will clearlyenable one skilled in the art to make and use the invention, anddescribes several embodiments, adaptations, variations, alternatives anduses of the invention, including what we presently believe is the bestmode of carrying out the invention. Additionally, it is to be understoodthat the invention is not limited in its application to the details ofconstruction and the arrangements of components set forth in thefollowing description or illustrated in the drawings. The invention iscapable of other embodiments and of being practiced or being carried outin various ways. Also, it is to be understood that the phraseology andterminology used herein is for the purpose of description and should notbe regarded as limiting.

A worm assembly 10 is shown generally in FIG. 1. The worm assemblyincludes a worm input shaft 12 having a spiraling worm thread 14 formedthereon. The input shaft 12 is connected to a prime mover or drive 16,such as a motor. The worm thread 14 meshes with the teeth 18 of a wormgear or wheel 20. An output shaft 22 extends from the center of the wormgear 20 to be rotated by the worm gear. The worm input shaft 12 and theworm gear 20 are contained within a housing 24, and the input and outputshafts extend from the housing to be connected to a driver 16 and adriven element (not shown). Although the driver 16 is shown beingconnected to the worm input shaft 12, it could alternatively beconnected to the worm gear 20 via the worm gear shaft 22, such that theworm gear 20 drives the worm shaft 12.

The input shaft 12 and output shaft 22 are sealed via seals 26 and 28,respectively. As best seen in the enlarged view of FIG. 2, the seal 26is positioned exteriorly of the input shaft bearing 30 which supportsthe input shaft 12. The bearing 30 is received in a bearing seat 32 inthe housing which surrounds an opening 34 through which the shaft 12extends. The output shaft is supported by a bearing (not shown) as well.The seal 26 extends between the surface defining the opening 34 and theshaft 12 and is an elastomeric seal.

The useful life of the worm gear box can be increased by enhancing theseal performance. Seal performance can be improved by selecting optimalmaterials, finishes, and/or coating selections for the worm, worm gear,shaft, and/or shaft sleeves. The selection of optimal materials,finishes and/or coatings results in increased seal life; less seal heatgeneration; lower torque and power losses; and less seal leakage. Thisoccurs by reducing friction and by preventing debris formation caused byadhesive and/or abrasive wear of the worm and gear; reducing thefrictional heat generation in the worm/worm gear mesh; reducing thefrictional heat generation in the seal interface; and reducing seal andshaft or sleeve wear caused by contact with each other or a third bodycontaminant.

One method to improve seal performance is to employ a hardened steelworm with a smooth, non-directional surface topography operating againsta standard bronze worm gear. The topographically enhanced worm wouldminimize debris formation by minimizing solid-solid adhesiveinteractions between the worm and worm gear. Such a configuration wouldpromote low mesh frictional losses, possibly allowing the use of a lowerviscosity lubricant. Reducing the amount of debris in the lubricant andthe heat generation from the mesh frictional losses would reduce theamount of seal wear, thus reducing seal leakage and extending seal life.

The steel used for the worm can be hardened by a sequence of heating toproduce austenite. Preferably, the heating sequence produces at least50% austenite in the steel. The worm can be heated in a furnace or by alaser, electron beam, magnetic induction or visible light. Afterheating, the steel can be quenched and tempered. The heated parts can bequenched in a hydrocarbon based or aqueous based liquid, air, partialvacuum or gas. Selections of the heating time and temperature, quenchingmedia selection, and tempering time and temperature are based upon thesteel composition and values of the desired properties such as hardnessand toughness. In one example, the worm would be made from wroughtAISI/SAE type 4140 steel. The steel would be austenitized at about 1575°F. (about 855° C.) for at least about one (1) hour, quenched into oil,and then tempered for about two hours at about 400° F. (about 204° C.)to produce a hardness of approximately 50 HRC throughout the part. Theworm could also be made of different types of steel and/or be heattreated in other manners.

Carbon gradients or carbon and nitrogen gradients can be developed onthe spiral threads of the worm. The shaft may also be allowed to developcarbon or carbon and nitrogen gradients as well, or masked-off toprevent formation of these gradients. Carbon concentration gradients canbe formed by carburizing the worm. The carburizing can be performed bygaseous carburizing, vacuum carburizing, ion carburizing, or packcarburizing. After carburizing, a nitrogen concentration gradient can bedeveloped in the worm using gaseous nitriding, vacuum nitriding, ionnitriding or salt bath nitriding. Alternatively, carbon and nitrogenconcentration gradients can be formed concurrently within the worm viacarbonitriding or nitrocarburizing of the worm. Carbonitriding can beperformed using gaseous carbonitriding, vacuum carbonitriding, ioncarbonitriding, or pack carbonitriding. Nitrocarburizing can beperformed using gaseous nitrocarburizing, vacuum nitrocarburizing, ionnitrocarburizing or salt bath nitrocarburizing. The concentration ofCarbon and/or Nitrogen would be maximized at the surface. Theconcentration(s) would decrease with increasing distance from thesurface. A portion of the worm, including the center, would have theoriginal composition. Illustrative carbon and nitrogen gradient profilesare shown in FIGS. 3 a-d. The profiles shown are not to scale and areillustrative only. That is, the actual profile may vary from theprofiles shown in the graphs of FIGS. 3 a-d.

In one example, the worm would be made from wrought AISI/SAE type 8620steel. The steel would be carburized in a furnace with acarbon-containing gas mixture such as methane and nitrogen at atemperature of about 1700° F. (about 925° C.) for about six (6) hours;reduced in temperature to about 1550° F. (about 845° C.) while reducingthe carbon potential; and then quenched in oil and tempered at about350° F. (about 177° C.) for about two (2) hours to produce a surfacehardness of approximately 60 HRC. The hardness would decrease withincreasing distance from the surface until reaching an essentiallyconstant value of approximately 30 to 45 HRC. The carbon content woulddecrease from ≧0.70 wt % C near the surface with increasing distancefrom the surface until reaching an essentially constant value ofapproximately 0.20 wt % C.

In another example, the worm would be made from wrought AISI/SAE 4140steel. The steel would be austenitized at about 1575° F. (about 855° C.)for at least about one (1) hour; then quenched in oil and tempered forabout two (2) hours at 1100° F. (about 595° C.) for about twenty-four(24) hours to produce a surface hardness of approximately 700 HKN (orabout 60 HRC). The hardness would decrease with increasing distance fromthe surface until reaching an essentially constant value ofapproximately 30 HRC.

In yet another example, the worm would be made from wrought AISI/SAE4140 steel. The steel would be austenitized at about 1575° F. (about855° C.) for at least about one (1) hour; then quenched into oil andtempered for about two (2) hours at 1100° F. (about 595° C.) to producea surface hardness of approximately 29 HRC. The worm would then benitrocarburized by immersion in a carbon and nitrogen containing saltbath at a temperature of approximately 1050° F. (about 525° C.) forabout five (5) hours to produce a surface hardness equivalent toapproximately 60 HRC. The hardness would decrease with increasingdistance from the surface until reaching an essentially constant valueof approximately 29 HRC.

The above examples are illustrative only. The worm can be made fromother types of steel and can be carburized, nitrocarburized, orcarbonitrided in other manners.

One method for enhancing the surface texture on the worm could bevibratory finishing. Other methods of providing the optimal surfacetexture on the worm include hard turning, honing, grinding, and rolling.Combinations of the noted texturing methods can also be used to enhancethe surface texture of the worm.

It should be noted that the portion of the shaft or shaft sleeve thatcontacts the seal may be need to be prevented from being topographicallymodified. The seal contacting area is typically plunge ground to lessthan or equal to 20 microinch arithmetical average surface roughness orRa without a continuous helical pattern (lead) or directionality.Topographical modification could reduce the surface roughness below theminimum limit and could eliminate the desirable surface finish patternfrom grinding. Hence, preferably, the input and output shafts or shaftsleeves are not topographically modified in the vicinity of where theseal will contact the shafts.

Another approach for improving seal performance would be to employ astandard bronze worm gear and a worm made from hardened steel which istopographically enhanced and has a tribological coating applied thereto.The coating can be applied only to the shaft or to both the spiralthread (teeth) and shaft. This second approach adds to the firstapproach the tribological coating. The worm steel is hardened andtopographically enhanced as noted above. The topographically enhancedand coated worm would minimize debris formation by minimizing adhesivetooth wear between the worm and worm gear and/or abrasive tooth wear ofthe worm gear while promoting low tooth mesh frictional losses. Thetribological coating would have a top functional layer comprised ofnanocrystalline metal carbides such as tungsten, titanium, and chromiumto name a few dispersed in an amorphous hydrogenated carbon matrix. Thesurface texturing of the worm is performed before the worm has beencoated with the tribological coating.

Yet another approach to improve seal performance would be to employ atribological coating on the shaft or a shaft sleeve where the seal rides(seal counter-face). A shaft sleeve is a separate hollow cylinder piecethat is placed on the outside diameter of the shaft. It will beunderstood that, throughout when a shaft is referred to, this will alsoencompass the use of a shaft sleeve in conjunction with the shaft. Thisapproach adds to the first or second approach, the use of the coating onthe input and/or output shafts. The tribological coating for the shaftor shaft sleeve would be comprised of a material that is chemicallycompatible with the elastomeric seal such that seal wear and heatgeneration is reduced at the shaft-to-seal interface therefore reducingseal leakage and extending seal life. The tribological coating wouldhave a thin solid functional layer comprised of primarily chromium andnitrogen elements in a dense microstructure. The shaft is typicallyplunge ground to less than or equal to 20 microinch arithmetical averagesurface roughness or Ra without a continuous helical pattern (lead) ordirectionality prior to coating.

The surface texture of the worm gear can be enhanced as well. As withthe worm spiral, enhancement of the worm gear can be accomplished byvibratory processing, peening, hard turning, honing, rolling andcombinations thereof.

Thus the gear teeth may receive one treatment and the seal counter-facesurface may receive a different treatment.

Four different approaches are outlined above: (1) the use of a hardenedsteel worm with a bronze worm gear; (2) the use of a hardened steel wormand a bronze worm gear with a tribological coating on one or both of theworm thread or spiral and the gear teeth; 3) the use of a hardened steelworm and a bronze worm gear with a tribological coating on one or bothof the input or output shaft or shaft sleeves; or (4) the use of ahardened steel worm with a bronze worm gear with a tribological coatingapplied to the worm thread, the gear teeth, the input and output shaftsor shaft sleeves, or combinations thereof. In each of the threeapproaches, the worm, worm gear, and input and output shafts can havesurface finishing treatments. Additionally, in each of the fourapproaches, carbon or carbon/nitrogen concentration gradients can beadded to the worm. As can be appreciated, each approach builds on theprior approach to further improve seal performance, thereby reducingseal leakage and extending seal life.

The method of the present invention will allow a designer to combinegear teeth and seal counter-face material, multiple surface finishes,and multiple coating enhancements to improve worm gear seal systemperformance and provide longer, leak-free seal life. The gear toothmaterial and surface treatments eliminate seal counter-face wear frominternally generated bronze debris from the gear teeth. Sealcounter-face treatments, which may be different than the gear toothsurface treatments, eliminate seal counter-face wear from externaldebris and reduces frictional heat. This system's synergistic approachallows one to achieve optimum performance results at a lower life cyclecost when compared to using stand-alone seal solutions.

As various changes could be made in the above constructions withoutdeparting from the scope of the invention, it is intended that allmatter contained in the above description or shown in the accompanyingdrawings shall be interpreted as illustrative and not in a limitingsense.

1. In a worm gear speed reducer assembly comprising a worm shaft havinga spiral thread; a worm gear having worm gear teeth; gear shaftextending from the center of the worm gear; the worm spiral and the wormgear being contained within a housing, and the worm and gear shaftsextending from the housing; bearings supporting the worm and gear shaftsand seals extending between an opening in the worm gear housing and theworm and gear shafts, a method for improving the attributes of a seal ofthe speed reducer assembly that is in contact with any part of a shaftor shaft sleeve of the speed reducer assembly during operation; themethod comprising: imparting a surface finish on one or both of the wormspiral and worm gear teeth to provide enhanced resistance to adhesivewear of the teeth which will minimize abrasive wear at the shaft-to-sealinterface;
 2. The method in claim 1 wherein the step of imparting asurface finish is performed by vibratory processing, peening, hardturning, honing, rolling, grinding or combinations thereof.
 3. Themethod of claim 2 wherein the tooth surface finish comprises of lessthan 8 microinch surface roughness (Ra).
 4. The method of claim 2wherein surface finish of the area of the shaft or shaft sleevecontacted by the seal comprises of less than or equal to 20 microincharithmetical average surface roughness (Ra) without any lead.
 5. Themethod in claim 1 wherein the worm is a steel worm; the methodcomprising a step of hardening the steel used for the worm; the step ofhardening the worm comprising heating the worm to produce at least someaustenite, quenching the worm and tempering worm.
 6. The method of claim5 wherein the step of heating the worm comprises heating the worm in afurnace or by a laser, electron beam, magnetic induction, visible light,or combinations thereof; and, the step of quenching being performed in ahydrocarbon based or aqueous based liquid, or in air, or in partialvacuum or in a gas.
 7. The method in claim 5 including a step ofdeveloping of a carbon concentration gradient within the worm by gaseouscarburizing, vacuum carburizing, ion carburizing, or pack carburizing.8. The method of claim 5 including a step of developing of a nitrogenconcentration gradient within the worm using gaseous nitriding, vacuumnitriding, ion nitriding, salt bath nitriding.
 9. The method of claim 5including a step of concurrently developing nitrogen and carbonconcentration gradients within the worm by gaseous nitrocarburizing,vacuum nitrocarburizing, ion nitrocarburizing or salt bath nitrocarburizing.
 10. The method in claim 5 including a step ofconcurrently developing carbon and nitrogen concentration gradientswithin the worm by gaseous carbonitriding, vacuum carbonitriding, ioncarbonitriding, or pack carbonitriding.
 11. In a worm gear assemblycomprising a worm having a worm shaft and a spiral thread; a worm gearhaving worm gear teeth; and a gear shaft extending from the center ofthe worm gear; the worm shaft and the worm gear being contained within ahousing, and the worm and gear shafts extending from the housing;bearings supporting the worm and gear shafts in the housing and sealsextending between an opening in the worm gear housing and the worm andgear shafts, the improvement comprising a method for improving theattributes of a worm gear seal that is in contact with any part of theworm gear shaft or shaft sleeve during operation; the method comprising:imparting a coating on one or both of the worm and gear shafts or shaftsleeves to provide enhanced resistance to adhesive and/or abrasive wearat the shaft-to-seal interface.
 12. The method in claim 11 and includinga step of imparting a surface finish to the shaft or shaft sleeves byvibratory processing, peening, vibratory processing, peening, peening,hard turning, honing, rolling, grinding, or combinations thereof. 13.The method of claim 12 wherein the surface finish of the area of theshaft or shaft sleeve contacted by the seal comprises of less than orequal to 20 microinch arithmetical average surface roughness (Ra)without any lead.
 14. The method of claim 11 wherein the worm is a steelworm; the method comprising a step of hardening the steel used for theworm; the step of hardening the worm comprising heating the worm toproduce at least some austenite, quenching the worm and tempering worm;the step of heating being performed in a furnace or by a laser, electronbeam, magnetic induction, visible light, or any other suitable hardeningprocess; the step of quenching being performed in a hydrocarbon based oraqueous based liquid; air, partial vacuum or gas.
 15. The method inclaim 14 including a step of developing of a carbon concentrationgradient within the worm by gaseous carburizing, vacuum carburizing, ioncarburizing, or pack carburizing.
 16. The method of claim 14 including astep of developing of a nitrogen concentration gradient within the wormusing gaseous nitriding, vacuum nitriding, ion nitriding, salt bathnitriding.
 17. The method of claim 14 including a step of concurrentlydeveloping nitrogen and carbon concentration gradients within the wormby gaseous nitrocarburizing, vacuum nitrocarburizing, ionnitrocarburizing or salt bath nitrocarburizing.
 18. The method in claim14 including a step of concurrently developing carbon and nitrogenconcentration gradients within the worm by gaseous carbonitriding,vacuum carbonitriding, ion carbonitriding, or pack carbonitriding. 19.The method of claim 11 wherein the coating is applied to the areacontacted by the seal on the input shaft or input shaft sleeve and/orthe output shaft or output shaft sleeve.
 20. The method of claim 19wherein the shaft and/or shaft sleeve coating comprises a thin solid topfunctional layer comprised of primarily chromium and nitrogen elementsin a dense microstructure.
 21. The method of claim 19 including a stepof imparting a surface finish to the shaft or shaft sleeves wherein thesurface finish is applied before the tooth and/or shaft and/or shaftsleeve coating is applied.
 22. The method in claim 11 including a stepof imparting a surface finish to the worm spiral and/or the gear teeth;the step of imparting the surface finish being performed by vibratoryprocessing, peening, hard turning, honing, rolling, grinding orcombinations thereof.
 23. The method of claim 22 wherein the surfacefinish applied to the gear teeth has less than about 8 microinch surfaceroughness (Ra).
 24. The worm gear assembly of claim 11 wherein a coatingis applied to the worm spiral and/or the gear teeth.
 25. The worm gearassembly of claim 24 wherein the tooth coating comprises a topfunctional layer comprised of nanocrystalline metal carbides dispersedin an amorphous hydrogenated carbon matrix.
 26. The method of claim 24including a step of imparting a surface finish to the worm spiral and/orthe gear teeth wherein the surface finish is applied before the toothcoating is applied.
 27. In a worm gear assembly comprising a worm havinga worm shaft having a spiral thread; a worm gear having worm gear teeth;and a gear shaft extending from the center of the worm gear; the wormspiral and the worm gear being contained within a housing, and the wormand gear shafts extending from the housing; bearings supporting the wormand gear shafts and seals extending between an opening in the worm gearhousing and the worm and gear shafts; the improvement comprising amethod for improving the attributes of a worm gear seal that is incontact with any part of the worm gear shaft or shaft sleeve duringoperation comprising: imparting a coating on one or both of the wormspiral and worm gear teeth to provide enhanced resistance to adhesivewear of the teeth which will minimize abrasive wear at the shaft-to-sealinterface.
 28. The method in claim 27 including a step of imparting asurface finish to the worm spiral and/or the gear teeth; the step ofimparting the surface finish being performed by vibratory processing,peening, hard turning, honing, rolling, grinding or combinationsthereof.
 29. The method of claim 28 wherein the surface finish appliedto the gear teeth has less than about 8 microinch surface roughness(Ra).
 30. The method in claim 27 wherein the worm is a steel worm; themethod comprising a step of hardening the steel used for the worm; thestep of hardening the worm comprising heating the worm to produce atleast some austenite, quenching the worm and tempering worm.
 31. Themethod in claim 30 including a step of developing of a carbonconcentration gradient within the worm by gaseous carburizing, vacuumcarburizing, ion carburizing, or pack carburizing.
 32. The method ofclaim 30 including a step of developing of a nitrogen concentrationgradient within the worm using gaseous nitriding, vacuum nitriding, ionnitriding, salt bath nitriding.
 33. The method of claim 30 including astep of concurrently developing nitrogen and carbon concentrationgradients within the worm by gaseous nitrocarburizing, vacuumnitrocarburizing, ion nitrocarburizing or salt bath nitrocarburizing.34. The method in claim 30 including a step of concurrently developingcarbon and nitrogen concentration gradients within the worm by gaseouscarbonitriding, vacuum carbonitriding, ion carbonitriding, or packcarbonitriding.
 35. The worm gear assembly of claim 27 wherein thecoating comprises a top functional layer comprised of nanocrystallinemetal carbides dispersed in an amorphous hydrogenated carbon matrix. 36.The method of claim 35 including a step of imparting a surface finish tothe worm spiral and/or the gear teeth; wherein the surface finish isapplied before the coating is applied to the worm spiral and/or the gearteeth.
 37. The method in claim 27 and including a step of imparting asurface finish to the shaft or shaft sleeves by vibratory processing,peening, vibratory processing, peening, peening, hard turning, honing,rolling, grinding, or combinations thereof.
 38. The method of claim 37wherein the surface finish of the area of the shaft or shaft sleevecontacted by the seal comprises of less than or equal to 20 microincharithmetical average surface roughness (Ra) without any lead.
 39. Themethod of claim 37 wherein the coating is applied to the area contactedby the seal on the input shaft or input shaft sleeve and/or the outputshaft or output shaft sleeve.
 40. The method of claim 39 wherein theshaft and/or shaft sleeve coating comprises a thin solid top functionallayer comprised of primarily chromium and nitrogen elements in a densemicrostructure.
 41. The method of claim 39 wherein the surface finish isapplied before the tooth and/or shaft and/or shaft sleeve coating isapplied.
 42. A worm gear assembly comprising a worm having a worm shaftand a spiral thread; a worm gear having worm gear teeth; an output shaftextending from the center of the worm gear; the worm spiral and the wormgear being contained within a housing, and the input and output shaftsextending from the housing; bearings supporting the worm and gear shaftsand seals extending between an opening in the worm gear housing and theworm and gear shafts; the improvement comprising: the worm being madefrom a hardened steel and the gear being made from a softer metal; thesurface of one or more of the worm spiral, gear teeth, worm shaft, andgear shaft having an enhanced surface texture to enhance the useablelife of the seals.
 43. The worm gear assembly of claim 42 wherein theseal contacting area has less than or equal to 20 microinch arithmeticalaverage surface roughness or Ra without a continuous helical pattern ordirectionality.
 44. The worm gear assembly of claim 42 wherein the toothsurface finish has a surface roughness (Ra) of less than 8 microinchsurface roughness (Ra).
 45. The worm gear assembly of claim 42 whereinthe worm has at least 50% austenite in the steel.
 46. The worm gearassembly of claim 42 wherein the worm has a surface hardness of about 50HRC to about 60 HRC.
 47. The worm gear assembly of claim 42 wherein theworm hardness decreases radially inwardly from the surface to a hardnessof about 30 HRC.
 48. The worm gear assembly of claim 42 wherein the wormhas a carbon concentration gradient, the carbon concentration beinggreatest at the surface and decreasing inwardly from the surface. 49.The worm gear assembly of claim 48 wherein the worm has a carbonconcentration of about 0.7 wt % at the surface, said concentrationdecreasing to about 0.2 wt %.
 50. The worm gear assembly of claim 48wherein the worm gear assembly also has a nitrogen gradient.
 51. Theworm gear assembly of claim 42 including a tribological coating appliedto one or more of the worm spiral, gear teeth, worm shaft and gearshaft.
 52. The worm gear assembly of claim 51 wherein the tribologicalcoating applied to the worm spiral and/or gear teeth has a topfunctional layer comprised of nanocrystalline metal carbides dispersedin an amorphous hydrogenated carbon matrix.
 53. The worm gear assemblyof claim 51 wherein the tribological coating applied to the worm shaftand/or gear shaft comprises a thin solid functional layer comprised ofprimarily chromium and nitrogen elements in a dense microstructure. 54.A worm gear assembly comprising a worm having a worm shaft and a spiralthread; a worm gear having worm gear teeth; an output shaft extendingfrom the center of the worm gear; the worm spiral and the worm gearbeing contained within a housing, and the input and output shaftsextending from the housing; bearings supporting the worm and gear shaftsand seals extending between an opening in the worm gear housing and theworm and gear shafts; the improvement comprising: the worm being madefrom a hardened steel and the gear being made from a softer metal; atribological coating applied to one or more of the worm spiral, gearteeth, worm shaft and gear shaft.
 55. The worm gear assembly of claim 54wherein the tribological coating applied to the worm spiral and/or gearteeth has a top functional layer comprised of nanocrystalline metalcarbides dispersed in an amorphous hydrogenated carbon matrix.
 56. Theworm gear assembly of claim 54 wherein the tribological coating appliedto the worm shaft and/or gear shaft comprises a thin solid functionallayer comprised of primarily chromium and nitrogen elements in a densemicrostructure.